1. Field of the Invention
The present invention relates to the retorting of oil shale. More specifically, the invention pertains to the retorting of low grade shale wherein at least part of the heat required in the retort is provided through combustion of residual carbonaceous material remaining on previously retorted shale and wherein shale fines in the retorted oil product are removed to facilitate downstream processing.
2. Description of the Prior Art
Vast natural deposits of shale in Colorado, Utah and Wyoming contain appreciable quantities of organic matter which decompose upon pyrolysis to yield oil, hydrocarbon gases and residual carbon. The organic matter or kerogen content of these deposits has been estimated to be equivalent to approximately 500 million cubic meters of oil. As a result of the dwindling supplies of petroleum and natural gas, extensive research efforts have been directed to develop retorting processes which will economically produce shale oil on a commercial basis from these vast resources.
In principle, shale retorting simply comprises heating the raw shale to an elevated temperature and recovering the vapors evolved. The process heat requirements may be supplied either directly or indirectly. Directly heated retorting processes rely upon the combustion of fuel in the presence of the oil shale to provide sufficient heat for retorting. Such processes result in lower product yields due to unavoidable combustion of some of the product and dilution of the product stream with the products of combustion. Indirectly heated retorting processes generally use a separate furnace or equilvalent vessel in which a solid or gaseous heat carrier is heated. The hot heat carrier is subsequently mixed with the shale to provide heat, thus resulting in higher yields while avoiding dilution of the retort products with combustion products.
In essentially all above-ground processes for the retorting of shale, the shale is first crushed to reduce the size of the shale to aid in materials handling and to reduce the time for retorting. Unfortunately crushing operations have little or no control over the breadth of the resulting particle size distribution, as this is primarily a function of the rock properties. Normally a considerable quantity of shale fines are produced through the initial crushing step.
As the shale is retorted, even more fines are generated since the shale particles lose strength, beome more friable and less resistant to degradation by attrition during the process.
Many processes (see U.S. Application Ser. No. 889,156 incorporated herein by reference) strip the evolved vapors from the retorted shale as an integral step of product recovery with steam or other gases. Unfortunately, the stripping fluid and evolved vapors also entrain quantities of the fines produced through the initial crushing operation or resulting from attrition in the retort. These fines create problems in downstream processing of the product and must be ultimately removed.
Certain of the more thermally attractive processes use the energy value of the residual carbon on the retorted shale as a source of energy. Typical processes illustrating this approach include U.S. Pat. No. 3,784,462 issued to G. W. Frick, U.S. Pat. No. 4,054,492 issued to R. Rammler et al and U.S. Application Ser. No. 889,156.
In U.S. Pat. No. 3,784,462 retorted shale particles are heated by combustion of its residual carbon to provide a hot heat transfer medium which is countercurrently contacted with raw shale.
In U.S. Pat. No. 4,054,492 spent shale is combusted in a lift pipe and mixed with raw shale in a horizontal distillation vessel.
In U.S. Application Ser. No. 889,156, hot spent shale, heated by combustion in air, and raw shale are introduced into an upper portion of an elongated vessel and pass downwardly therethrough countercurrent to an upwardly flowing stripping gas. A product stream of hydrocarbon vapors, stripping gas and entrained solids are recovered overhead while the retorted solids and spent shale carrier are withdrawn from the bottom of the retort.
With most of the processes the shale fines removal is accomplished by conventional techniques such as batteries of hot filters, cyclone separators, electrostatic precipitators and the like, thus adding to the plant capital cost.